Hybrid graphene - silver nanoantenna to control THz emission from polar quantum systems.

Fluorescent light emission from atomic quantum systems routinely occurs at the illumination frequency. If the system is polar, an additional fluorescence peak appears at a much lower Rabi frequency, which scales with the illumination field amplitude. This opens the possibility of spectrally controlling the emission, promising tunable coherent radiation sources. However, typically the emission occurs in the MHz to GHz regimes, and its intensity from a single quantum system is relatively low. Here, we propose a hybrid nanoantenna combining noble-metal and graphene elements, exploited for an unusual goal: The silver elements spectrally tune the emission frequency of the molecule and shift it to the THz band, where novel sources of coherent radiation are still desired. Additionally, the graphene elements are used to plasmonically enhance the emission intensity. Their tunability allows for adjustment of the operational frequencies of the device to the illumination conditions and to counteract the fluctuations related to the field modulations in space. All these features are discussed based on the real-life example of a polar molecule of barium monofluoride (BaF).

Superluminal light propagation in a three-level ladder system.

Superluminal light propagation is typically accompanied by significant absorption that might prevent its observation in realistic samples. We propose an all-optical implementation exploiting the two-photon resonance in three-level media to overcome this problem. With several computational methods, we analyze three possible configurations of optically-dressed systems and identify an optimal configuration for superluminal propagation. Due to the far-detuned operating regime with low absorption, this scenario avoids the usual need for population inversion, gain assistance or nonlinear optical response. Our analysis covers a broad parameter space and aims for the identification of conditions where significant pulse advancement can be achieved at high transmission levels. In this context, a figure of merit is introduced accounting for a trade-off between the desired group-index values and transmission level. This quantity helps to identify the optimal characteristics of the dressing beam.

Propagation of optically tunable coherent radiation in a gas of polar molecules.

Coherent, optically dressed media composed of two-level molecular systems without inversion symmetry are considered as all-optically tunable sources of coherent radiation in the microwave domain. A theoretical model and a numerical toolbox are developed to confirm the main finding: the generation of low-frequency radiation, and the buildup and propagation dynamics of such low-frequency signals in a medium of polar molecules in a gas phase. The physical mechanism of the signal generation relies on the permanent dipole moment characterizing systems without inversion symmetry. The molecules are polarized with a DC electric field yielding a permanent electric dipole moment in the laboratory frame; the direction and magnitude of the moment depend on the molecular state. As the system is resonantly driven, the dipole moment oscillates at the Rabi frequency and, hence, generates microwave radiation. We demonstrate the tuning capability of the output signal frequency with the drive amplitude and detuning. We find that even though decoherence mechanisms such as spontaneous emission may damp the output field, a scenario based on pulsed illumination yields a coherent, pulsed output of tunable temporal width. Finally, we discuss experimental scenarios exploiting rotational levels of gaseous ensembles of heteronuclear diatomic molecules.

Enhancement of and interference among higher order multipole transitions in molecules near a plasmonic nanoantenna.

Spontaneous emission of quantum emitters can be modified by their optical environment, such as a resonant nanoantenna. This impact is usually evaluated under assumption that each molecular transition is dominated only by one multipolar channel, commonly the electric dipole. In this article, we go beyond the electric dipole approximation and take light-matter coupling through higher-order multipoles into account. We investigate a strong enhancement of the magnetic dipole and electric quadrupole emission channels of a molecule adjacent to a plasmonic nanoantenna. Additionally, we introduce a framework to study interference effects between various transition channels in molecules by rigorous quantum-chemical calculations of their multipolar moments and a consecutive investigation of the transition rate upon coupling to a nanoantenna. We predict interference effects between these transition channels, which allow in principle for a full suppression of radiation by exploiting destructive interference, waiving limitations imposed on the emitter's coherence time by spontaneous emission.